| Loess is porous and disintegrating.When water seeps through the soil,it will produce osmotic force,which reduces the strength and increases the deformation of the soil.The loess slope located in seasonal frozen soil area will also be affected by freezethaw cycles and salt erosion,which will make the soil deformation even worse,and even cause landslides and other natural disasters.Therefore,permeability is one of the important engineering mechanical properties of loess.The change of permeability characteristics of saline loess caused by freeze-thaw cycles is a complex problem,and there are few related studies.Based on this,this paper,started from both micro and macro aspects,reveals the mechanism of permeability change of undisturbed saline loess under freeze-thaw cycles by conducting indoor nuclear magnetic resonance(NMR)tests and triaxial permeability tests.In the micro aspect,this paper took the undisturbed loess in Xi’an as the research object and carried out the nuclear magnetic resonance tests.The experimental results show that the soil porosity increases with the increase of freeze-thaw times,and the increase range decreases gradually,showing an exponential growth;the soil porosity increases with the increase of salt content,and the increase range increases gradually.After 10 freeze-thaw cycles,the internal pore diameters of soils are still mainly 0.01-0.1and 0.1-10 microns.The sum of the two volumes accounts for about 70% of the total pore volume.The first two freeze-thaw cycles make the fractal dimension of soil decrease temporarily;with the increase of freeze-thaw cycles,the fractal dimension begins to increase,and the increase range gradually decreases.In the macro aspect,this paper took the undisturbed loess in Xi’an as the research object,and carried out apparent analysis and triaxial permeability test.The apparent analysis shows that the variation of surface crack rate with freeze-thaw times and salt content is the same as that of porosity.The triaxial permeability test results show that the permeability coefficient of the soil gradually increases with the increase of the number of freeze-thaw cycles,and the increase rate gradually becomes smaller.It increases exponentially,which is same as the variation of porosity.The permeability coefficient of the soil gradually decreases with the increase of confining pressure,and the decreasing extent decreases gradually,which decreases exponentially.If the soil has not undergone freeze-thaw cycles,its permeability coefficient tends to decrease with the increase of salt content;if the soil has undergone freeze-thaw cycles,its permeability coefficient gradually increases with the increase of salt content,and the increase range increases gradually,which is same as the variation of porosity.Finally,based on the basic data of penetration tests,this paper used BP neural network to predict the variation of permeability coefficient of undisturbed saline loess during freeze-thaw processes,and the test results show that the prediction accuracy is good. |
